Various methods of elevating the sensitivity of photographic materials without enlarging the grain size of the silver halide grains in the emulsions which constitute the materials have heretofore been proposed. One example is a photographic material containing an internal latent image emulsion for forming a latent image on the inside of the grain by exposure, in which the inside of the grain is chemically sensitized. For instance, U.S. Pat. Nos. 2,696,436, 3,206,313, 3,917,485, 3,979,213 and 4,623,612 and JP-B-43-29405 and JP-B-45-13259 (the term "JP-B" as used herein means an "examined Japanese patent publication") describe techniques for producing silver halide photographic emulsions having a large internal sensitivity and silver halide photographic materials containing such emulsions, by dipping a silver halide emulsion-coated sample in an AgNO.sub.3 solution or in a silver halide solvent, or by sensitizing a silver halide emulsion by chemical sensitization followed by Ostwald ripening during the step of forming it, or by adding an aqueous AgNO.sub.3 solution and an aqueous soluble halide solution to a silver halide emulsion during the step of forming it. It is asserted in these references that the silver halide photographic materials containing the emulsions thus prepared have excellent photographic characteristics. However, most of the photographic materials produced by the proposed techniques may express an excellent photographic sensitivity only to particular internal developers but do not express a sufficient photographic sensitivity to ordinary developers which do not contain an unusually large amount of a silver halide solvent such as potassium iodide or sodium thiosulfate.
In order to overcome this drawback, U.S. Pat. No. 3,966,476 describes an emulsion capable of forming a latent image by exposure. The latent image may be opened to the surface of the silver halide grain in the emulsion so that it may be developed with a surface developer. However, the emulsion described therein is not a so-called internal latent image emulsion and therefore it does not sufficiently display the excellent photographic characteristics found in an internal latent image emulsion.
U.S. Pat. Nos. 4,839,268 and JP-A-63-264740 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describe a technique relating to an internal latent image emulsion which is capable of expressing a high sensitivity to a developer having a broad range and a photographic material containing an emulsion of that type. These references state that an emulsion which expresses a sufficient photographic sensitivity to an ordinary photographic developer should have a structure so that the position for forming a latent image by exposure is within a specific distance of the surface of the emulsion grain and so that a latent image of some degree is formed also on the surface of the grain. However, the present inventors have found that in the emulsions disclosed in these references, the part which is chemically sensitized is very near the surface of the grain so that the emulsions are fogged too often and the storage stability of the photographic materials between the time of their manufacture and their use is bad.
Additionally, various reports have disclosed compounds having the function of preventing elevation of fog during storage. For instance, the antifoggants described in Research Disclosure RD 17643, page 24 and Research Disclosure RD 18716, page 649, as well as palladium complex salts, are known. Palladium complex salts appropriate for this purpose include those described in U.S. Pat. Nos. 2,448,060, 2,472,627, 2,472,631, 2,566,245, 2,565,245, 2,598,079, 2,953,455, 4,092,171 and 4,102,312, and JP-A-60-80847. However, these studies disclose only surface latent image emulsions which can not express a sufficient sensitivity or internal latent image type emulsions which can not be developed by ordinary negative developers. Consequently, there is no effective technique for reducing fog of an emulsion containing chemically sensitized grains at a position near the surface of the grain.